1. Prior Art
The prior art is believed to be best exemplified by the following patents and literature references:
______________________________________ Hochwalt et al 2,168,160 8/39 Hochwalt et al 2,282,518 5/42 Ball et al 2,997,466 8/61 Webster 3,216,839 11/65 Ball et al 3,048,576 8/62 Ball et al 3,223,697 12/65 Herrick et al 3,454,508 7/69 Enkvist 3,864,291 2/75 Johansson 4,113,542 9/78 ______________________________________
Merewether, "The Precipitation of Lignin from Eucalyptus Kraft Black Liquors", TAPPI, Vol. 45, No. 2, pp. 159-163, February, 1962. PA0 Dolenko et al, "Resin binders from Kraft Lignin", Forest Products Journal, Vol. 28, No. 8, pp. 41-46, August, 1978. PA0 Forss et al, "Finnish Plywood, Particleboard, and Fiberboard Made With A Lignin-Base Adhesive", Forest Products Journal, Vol. 29, No. 7, pp. 39-43, July, 1979.
2. Field of the Invention
This invention is related to lignin and lignin products derived from black liquor.
Lignin is derived from wood as a by-product in the pulping process and as an abundant, natural and renewable product, has tremendous potential for many industrial uses as replacements for increasingly scarce and expensive petroleum based materials. Lignin has found limited utilization commercially, however, because it is complex chemically and physically and its characteristics have been found to vary considerably, depending on many factors including variations in pulping conditions, the kind of wood being pulped, the conditions under which it is recovered, etc.
It is known that lignin obtained as a by-product in alkaline pulping processes contains both aliphatic and aromatic hydroxyl groups which are attractive sites for chemical modifications. Because of the similarity of the hypothesized chemical structure of alkali lignin to phenolic resins, lignin has been proposed as a replacement for phenol in resins.
One such technological area where replacement of phenol by lignin has been suggested is in the plywood industry where lignin has been proposed as at least a partial substitute for expensive petrochemicals such as phenol in phenol-formaldehyde resins employed as adhesives. Such resins, commonly designated P/F resins, are made by condensing phenol with formaldehyde in water using sodium hydroxide catalyst to make at least 35% to 45% solids resin solutions.
Various workers in the art have proposed various means for employing lignin products as substitutes or extenders for phenolic reactants. For example, Dolenko et al, "Resin Binders From Kraft Lignin", Forest Products Journal, Vol. 28, No. 8, pp. 41-46, August, 1978; Johansson, U.S. Pat. No. 4,113,542; Forss et al, "Finnish Plywood, Particleboard and Fiberboard Made With A Lignin-Base Adhesive", Forest Products Journal, Vol. 29, no. 7, pp. 39-43, July, 1979; Hochwalt et al, U.S. Pat. Nos. 2,168,160 and 2,282,518 are each directed to aspects of such reactions. Webster, U.S. Pat. No. 3,216,839 is directed to replacement of phenolic components in the production of methylolated resole resins.
One problem that is not addressed to any significant extent by such workers in the art is the extreme difficulty met in providing high solids, liquid lignin solutions. For many of the applications in which it is desired to replace or extend phenols with lignin products, manufacturers require such lignin solutions which are not readily available. This requirement has several aspects. Many resin manufacturers are not equipped to handle powders and therefore require lignin product in liquid form. Additionally, in the production of certain P/F resins, during the condensation of phenol and formaladehyde, typically 1.0 part of water is generated by every 3 parts of formaldehyde in the condensing mixture so that it is necessary to employ low viscosity lignin solutions having high solids content i.e., of the order of at least 40% solids, to avoid introducing excessive amounts of water into the system.
The problem of the lignin supplier in meeting these requirements resides in the solubility characteristics of lignin as well as in the characteristics of lignin solutions. For example, lignin and lignin products derived from alkaline wood pulping black liquors are damp granules, not liquids, at 40% to 50% or higher solids and are usually dried to give powders. Such damp granules or powders are insoluble in water but may be dispersed in water with alkali to make solutions containing up to about 35% lignin and less than 40% solids. At higher concentrations of lignin or at higher solids, such alkali-water solutions become too viscous to handle. Additionally, it requires at least about 10% sodium hydroxide, based on the weight of the lignin, to make such solutions. Although this 10% sodium hydroxide on lignin level is less than the total amount of sodium hydroxide used as a catalyst in the phenol/formaldehyde condensation, some resin producers prefer to start the condensation reaction with little or no alkali and to add the necessary alkali incrementally to control the condensation. For these users, the 10% sodium hydroxide on lignin level will render sodium hydroxide-lignin solutions in water unfit for their requirements.
Hochwalt et al U.S. Pat. Nos. 2,168,160 and 2,282,518 describe solutions of lignin in phenol. The solutions are prepared by heating the lignin on a steam bath in the presence of phenol with stirring. Air dried lignin and lignin in its wet condition as originally obtained are disclosed to be soluble. Satisfactory results are said to be obtained in accordance with the processes described when the lignin contains approximately 80% by weight of water. No upper limit on solids content is mentioned other than what would be encompassed in "air-dried lignin". This term, however, encompasses materials having solids content varied over a considerable range depending on the material that is being air dried, the method and conditions of the air drying and the duration of the air drying. In any event, patentee makes no distinction between the solubility characteristics of lignins at various solids content.
To the contrary, we have found that the solubility of lignin is related to a number of factors including the lignin source, the history of the black liquor from which it is derived, and the amount of lignin solids put in solution. For example, lignin from aged and oxidized black liquor is more difficult to dissolve and keep in solution than lignin obtained from fresh black liquor. We have found that solution stability problems may be encountered with solutions having solids content within the range below about 75% solids resulting in unstable solutions that separate into two phases. Moreover, the nature of the instability has been found to be different with solutions having solids content below about 40-45% lignin solids when compared to solutions having solids content above about 40-45% lignin solids. For example, with solutions of lignin from aged black liquor with a solids content below about 40-45%, such as the 20% lignin solids of the Hochwalt et al patents, the solutions separate into two phases so rapidly that the mixtures cannot be stored and dispensed as a uniform composition and therefore have no commercial value. With solutions of lignin from aged black liquor in the 40-75% lignin solids range, some lignins are readily soluble and stay in solution for extended periods without any separation while other lignins form unstable solutions that separate into phases. The unstable solutions encountered in this solids range, however, separate so slowly that the mixtures can be transported and used without problem.
Additionally, low solids materials are not suitable for replacing phenol at the desired 20-50%, or higher, replacement level. For example, for the desired replacement level, solutions containing less than about 40-45% solids cannot be used in reactions designed to produce 35-45% P/F resins due to the water that is generated during the reaction. The resulting resins cannot be concentrated by evaporating water because additional heating advances the resins beyond the desired stage of cure. Therefore, solutions containing only 20% lignin solids would introduce too much water into the system for some producers of P/F resins.
Webster, U.S. Pat. No. 3,216,839 provides solutions of lignin designed for use in forming laminating resins of the A-stage resole type by dissolving lignin in methanol, ethanol or isopropanol containing not more than 10% by weight of water when methanol is the solvent; not more than 30% water with ethanol; and not more than 20-35% water with isopropanol. Since very little water can be tolerated in these systems, the lignin must be dried. Additionally and significantly, the alcohols employed are not components for the P/F condensation reaction.